Recent times have seen a tremendous worldwide escalation of activities towards providing more-and-more people with broadband access to the internet and other electronic information sources. Existing telephone and cable-television networks have been ‘hot-rodded’ to provide a privileged few the ability to retrieve digital information at a few hundred kilobits or a few megabits per second. However it is widely recognized that in order to make an information network that is truly interesting, relevant, and most importantly capable of delivering commercially-viable services, it needs to reach many more consumers with even higher bandwidth. Existing access networks, designed for cable television and wired or wireless telephone service, are not practically suited for reaching these levels. To address the anticipated demands, access providers (typically phone or cable companies) have begun planning and installing new fiber-optic access networks with fiber reaching from the central office to, or very near to, the consumer premises.
By a significant margin, the predominate architecture of these emerging fiber-optic networks is the Passive-Optical-Network, or PON. The term ‘passive’ here refers to the fact that between the Optical Line Termination (OLT) at the access providers central office (CO), and the Optical Network Unit (ONU), for instance on the side of the customers house, the fiber-optic network has no powered or dynamic components.
In typical usage presently, a PON transports two or three data streams between the CO and the consumer. There is a digital data stream going from the CO to the consumer (‘downstream’ data), a digital data stream from the consumer back to the CO (‘upstream’), and in some cases a hybrid analog/digital downstream carrying multi-channel video (i.e., the cable TV signal). Commonly, each of these streams is transported at a different wavelength so they can be more readily distinguished by the OLTs and ONUs. Typically the downstream (OLT to ONU) data would be transmitted at about 1490 nm (S-Band), the upstream (ONU to OLT) data at about 1310 nm (O-Band), and the video downstream at about 1550 nm (C-Band). There is also ongoing consideration to include additional wavelengths within the domain of about 1250 nm to 1625 nm to further increase the capacity of the network to transport digital streams.
The segments of the PON are invariably shared so that each OLT services several ONUs. The OLT can only service one ONU at a time for each digital stream (downstream digital broadcast is possible, but is not a common operation). The video stream is typically a free-running broadcast from the OLT to all the ONUs. The ONUs are synchronized to signals in the network so that at any given moment no more than one is communicating to the OLT. The downstream data is tagged such that only the ONU for which it is intended will forward the data into the premise network of that customer. Sharing is accomplished by splitting the fiber network into several branches within the PON using optical splitters, which passively divide the optical power evenly among all the downstream branches of that segment. Upstream signals passing through the splitter are reduced in power by the balance ratio, but are only carried upstream to the OLT and do not return downstream to the other ONUs.
Optical splitters for PONs typically service a modest number N of branches (i.e. 32) and may have one input (a 1×N splitter) or two inputs (a 2×N splitter). The 2×N variety of splitter is used when it is desired to combine the function of the splitter and a wavelength-agnostic service multiplexer (i.e. to combine digital downstream and video without regard for their individual wavelengths), or to simply accommodate the possibility of adding another service in the future.
The greatest expense in installing a new fiber-optic access network is the ‘trenching’ cost, or getting the transport components (chiefly fiber and splitters) installed from point-A (i.e., OLT) to all points-B (i.e., ONUs). As such, there is strong motivation to assure that the installed transport network is as adaptable as possible, and that it can be used for as-yet unspecified future network needs. The capacity of the fiber-optic network itself is much greater than is utilized by present schemes, and as long as it is kept adequately generic, it can be used for future higher-bandwidth schemes without needing to entrench a new network. Here the qualification ‘adequately generic’ primarily means transmission behavior is independent of wavelength over a range from 1.25μ (1250 nm) to 1.65μ (1650 nm). This means that it is highly desired that the fiber and other optical components between the OLT and the ONU's be wavelength insensitive over about 30% fractional bandwidth. Herein, for convenience but not based on any established convention, this is referred to as the ‘ultra-broad’ wavelength range.
For 1×N and 2×N splitters where N is 8, 16, 32, 64, or 128, the preferred splitter technology is Planar Lightwave Circuits. For N equal 2 or 4, fused biconic fiber splitters may also be competitive, depending on cost/performance requirements. For N not a power of two or larger than 128, splitter technologies have not been well investigated, so such values of N are not called for.
A Planar Lightwave Circuit, or interchangeably Printed Lightwave Circuit, (PLC) is an optical waveguide system fabricated on the surface of a substrate, commonly by means that can be closely compared to the replication processes used in manufacturing integrated electronic circuits. Even as PLCs become increasingly complex and sophisticated they remain, just as in electronic integrated circuits, primarily composed from a handful of basic circuit elements. One of the fundamental waveguide circuit elements is the four-port mixer. Each of the ports of such a device can fundamentally be used for optical signals either directed into or extracted out of the mixer, or even in both directions simultaneously, with various resultant behaviors. However, the four-port optical mixer is almost invariably characterized as a device with two ‘input’ ports and two ‘output’ ports with the implicit knowledge that the other modes of behavior can be derived from this specification. When the behavior of the four-port mixer is specified in this manner, it is commonly called a 2×2 coupler or a 2×2 splitter. When each of the coupler ‘outputs’ are connected to the input port of a 1×(N/2) splitter circuit element, the composite circuit provides the function of a 2×N splitter.
1×N splitters, where N is a power of two, can be made by cascading 1×2 splitters or 2×2 splitters. Three-port 1×2 splitters that provide sufficiently uniform division of power over the ultra-broad wavelength range are known in the art, and can be manufactured in a variety of non-trivial, but well demonstrated methods. The first stage of a 2×N splitter must be a four-port 2×2 device. Therefore, to have the capability to produce a wavelength-insensitive 2×2 splitter it is necessary, and generally sufficient to provide the capability to produce a wavelength-insensitive 2×N splitter.
In a 2×2 optical splitter, light entering into either of two input ports emerges from two output ports. Herein, input and output are terms used to identify particular ports and are not meant to imply a particular direction for the propagation of light. For example, in a 2×2 optical splitter, light may enter an output port and emerge from the input ports. Alternatively, light may enter both input ports simultaneously.
The most common 2×2 waveguide splitter circuit element is the resonant directional coupler. This simple directional coupler alone is not suitable as the input stage for a 2×N splitter due to it's wavelength dependence. A resonant directional coupler typically exhibits a ±½-dB stable balance ratio over only about 3% fractional bandwidth.
Another type of 2×2 splitter, known as an adiabatic 2×2 splitter, is known to be wavelength insensitive over a much greater wavelength range provided that it is sufficiently long. This type of splitter is generally not suitable in 2×N splitters for PONs because the extremely shallow converging tapers needed are difficult to manufacture consistently and sufficient device length required to get 30% fractional bandwidth is many times longer than a resonant directional coupler and the overall 2×N splitter size becomes too large to fit in the desired package size and too costly to manufacture. Furthermore, known methods of reducing the size of standard optical-waveguide circuit elements do not provide size reduction for adiabatic devices, and in fact often require an increase in the size of adiabatic devices. There does not seem to be any readily available prospects for practical adiabatic 2×2 splitters as a commodity for the deployment of commercial-scale PON networks.